Volume changes in hydrogels subjected to finite deformations

Constitutive equations are derived for the elastic response of hydrogels under an arbitrary deformationwith finite strains. An expression is proposed for the free energy density of a hydrogel based on the Floryconcept of a network of flexible chains with constrained junctions whose reference configuration differsfrom the initial configuration of a fully swollen gel. Adjustable parameters in the stress–strain relationsare found by fitting observations on poly(acrylamide) and gellan hydrogels under uniaxial tension andcompression. The effect of elongation ratio on osmotic Poisson’s ratio is examined numerically.